Internal Combustion Engines (ICE) are often called upon to generate considerable levels of power for prolonged periods of time on a dependable basis. Many such ICE assemblies employ a supercharging device, such as an exhaust gas turbine driven turbocharger, to compress the airflow before it enters the intake manifold of the engine in order to increase power and efficiency.
Specifically, a turbocharger is a centrifugal gas compressor that forces more air and, thus, more oxygen into the combustion chambers of the ICE than is otherwise achievable with ambient atmospheric pressure. The additional mass of oxygen-containing aft that is forced into the ICE improves the engine's volumetric efficiency, allowing it to burn more fuel in a given cycle, and thereby produce more power.
A typical turbocharger employs a central rotor shaft that transmits rotational motion between an exhaust-driven turbine wheel and an air compressor wheel. Such a rotor shaft is generally supported inside a center housing by thrust and journal bearings which are lubricated and cooled by engine oil and frequently receive additional cooling from specially formulated engine coolant. The exhaust gases that drive the turbine are prevented from entering the center housing by piston ring seals.
Turbochargers generally include a turbine housing for directing exhaust gasses from an exhaust inlet to an exhaust outlet across a turbine rotor. The turbine rotor drives a shaft journaled in a center housing section. A compressor rotor is driven on the other end of the shaft. The compressor rotor is housed in a compressor housing which directs air from the air filter into the compressor and out to the charge air cooler. The center housing bearing cavity with protected from the exhaust gases on the turbine side and the compressed air from the compressor side by piston ring seals.
Crankcase oil is commonly used to lubricate the rotating bearing interfaces as well as the thrust surfaces that limit axial excursions of the rotor shaft. Temperatures above 800° C. can occur in the exhaust gas turbine in the case of Diesel engines and above 1,000° C. in the case of Otto-cycle engines. Heat migrating from the turbine housing and turbine wheel into the shaft and center housing raise the temperature high enough to degrade or “coke” the oil that comes in contact with the rotor shaft and center housing adjacent to the turbine stage. This built up coked oil may interfere bind between the shaft shoulder adjacent to the turbine seal and the center housing. This binding restricts shaft rotation resulting in poor turbocharger boost performance.
As indicated, coking is an on-going issue with turbochargers given the very high operating temperatures. More specifically, heat from the exhaust gas tends to be conducted along the turbine rotor. The turbine rotor is affixed to the turbocharger shaft and a turbine seal is implemented at the joint between the turbine rotor, the shaft and the center housing. As lubricating oil passes through the narrow gap between the turbine rotor and the bearings it is heated to an elevated temperature as the lubricating oil contacts the heated shaft proximate to the turbine rotor. Accordingly, as the lubricating oil subsequently contacts the shaft which is heated by the turbine housing, coking is likely to occur. Accordingly, there is a need for a simple, low cost and effective means to prevent coking in the center housing of a turbocharger.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. Accordingly, there is a need for an improved turbocharger which reduces coking at the turbine shaft adjacent to the seal.